Not applicable.
1. Field of the Invention
The present invention relates to polyimide compositions useful in optical waveguide structures and subsystems. More particularly, the invention relates to a polyimide film and process for making the film that are used as cladding or core in polymeric optical waveguides. Still more particularly, the polyimide cladding or core of the present invention provides high optical transparency, low birefringence, resistance to solvents, strong adhesion to solid substrates, and a high thermal stability in passive optical films.
2. Description of the Related Art
Electronic information transfer systems distribute data through electrical signals. Optical information transfer systems distribute data at high rates through photonic signals. Optical systems use optical waveguides to carry the data. Passive optical waveguides connect optical information processing devices. A glass optical fiber is a passive optical waveguide, for example. A ribbon of polymer on a glass substrate can also be a passive waveguide. Passive waveguides are used to split, combine, couple and route optical signals. These optical waveguides typically comprise a transparent core and a transparent cladding surrounding the core, with the cladding material generally having a lower index of refraction than the core material.
Desirable characteristic for optical waveguides include high optical transparency, low birefringence, resistance to solvents, strong adhesion to solid substrates, and a high thermal stability. Low optical transmission loss, including low optical absorbance and scattering, allow the optical waveguide to transfer data efficiently over large distances.
Birefringence is a measure of the difference in index of refraction between the two orthogonal polarizations that normally exist in the waveguide. Birefringence may result, in part, from the chemical structure of the polymer used in the waveguide cladding or core. For example, a linear (para substituted) chemical structure in the polymer backbone may result in orientation of the polymer chains in the plane of the film causing increased birefringence. Large birefringence is generally detrimental to waveguide performance, causing the two polarizations to have different properties, such as mode size and propagation velocity. For example, a mode size mismatch between the waveguide and the optical fiber reduces the efficiency of coupling. A difference in propagation velocity leads to dispersion of the optical signals and places an upper bound on the rate at which data may be effectively transmitted through the device. Birefringence is measured as a continuous scale with 0.0 being the ideal. Birefringence is a measure of the optical anisotropy of the waveguide. It also is normally desired to minimize the difference in birefringence between the core and cladding materials in planer waveguides. Polymer chains in flat waveguide configurations are often oriented in the plane of the film and this generally leads to negative birefringence in the film. High birefringence is normally undesirable for optical waveguides and may contribute to high optical scattering losses and dispersion. Polyimides are of interest because of their high thermal stability. Accordingly, polyimides having relatively low birefringence are especially useful in planar waveguide applications. Since polyimides generally have a stiff backbone, low birefringence is difficult to achieve.
Optical waveguides should possess high thermal stability, which permits the waveguide to endure the electronics packaging and assembly processes used in manufacturing. The optical waveguide material should also allow etching (wet or dry) of a rib in the core or of a channel in the cladding having smooth sidewalls. Additionally, it is desirable to overcoat multiple polyimide layers without degradation of the previous layers from the solvent used in the added overcoat layers.
Several patents have disclosed polyimide structures in waveguide applications. U.S. Pat. No. 5,649,045 discloses polyimide structures having increased birefringence though the use of BPDA (biphenyl dianhydride). U.S. Pat. No. 5,317,082 discloses photodefinable photosensitive copolyimides and waveguide structures thereof having a 6FDA/BTDA dianhydride component, an aromatic diamine component having bulky methyl groups ortho to the amine, and a fluorinated co-diamine component to reduce birefringence. U.S. Pat. Nos. 5,598,501 and 5,572,619 disclose electron-beam irradiation of the film to define the core region of the waveguide. U.S. Pat. No. 5,344,916 discloses polyimide and copolyimide films having negative birefringence for use in liquid crystal displays, and a method for increasing the negative birefringence by increasing the amount of PMDA (pyromellitidianhydride) in the dianhydride component. None of these patents discloses the advantages of the substantially meta-linked polyimide composition for low birefringence of the present invention.
The present invention provides an optical waveguide subsystem comprising at least one cladding in contact with at least one core, wherein the cladding has a refractive index less than the refractive index of the core, and, at least one of the cladding or the core comprises a crosslinked polyimide that is substantially meta-linked and has an absolute birefringence of from about 0.01 to about zero.
The present invention also provides a process for fabricating an optical waveguide subsystem comprising the steps of (a) forming a polyimide from about 1 mole percent to about 15 mole percent of a first diamine, wherein the first diamine possesses a crosslinkable site, from about 35 mole percent to about 49 mole percent of at least one additional diamine, wherein the combined first and additional diamines possess a substantial amount of meta-linking configuration and the combined mole percent of the first and additional diamines equals approximately 50 mole percent, and approximately 50 mole percent of a dianhydride; (b) forming a solution of from about 20 weight percent to about 50 weight percent of the formed polyimide with from about 50 weight percent to about 80 weight percent of a solvent, and optionally from about 0.5 to about 10 weight percent of a crosslinking agent; (c) filtering the formed solution through a submicron filter; (d) spin-coating the filtered solution at from about 200 rpm to about 8000 rpm to form a film on a substrate; and, (e) baking the film at a final temperature which is the higher of from about 5xc2x0 C. to about 30xc2x0 C. above the glass transition temperature of the polyimide or approximately about 220xc2x0 C. for time period of from about 30 minutes to about 240 minutes. The polyimide may optionally be photocrosslinked.
The present invention provides crosslinked polyimide structures that are substantially meta-linked in optical waveguides for low birefringence and controllable refractive index properties. Meta-linked refers to the bond placement along the backbone of the polyimide, described below.
The present invention provides optical waveguide subsystems and a process for making them. The optical waveguide subsystem comprises at least one cladding in contact with at least one core. Either the cladding or core, or both, comprises a crosslinked polyimide that is substantially meta-linked along the backbone. The cladding has a refractive index less than the refractive index of the core. The polyimide cladding, core or preferably both, has a low birefringence of from about 0.01 to about zero. With the cladding in contact with the core, additional cladding and/or core components may be included in the present invention as needed for a given purpose of the optical waveguide device.
The present invention comprises a cladding or core component that comprises a substantially meta-linked polyimide. The polyimide of the present invention is formed from the combination of diamines and dianhydride compounds. In a preferred embodiment, the crosslinked polyimide comprises approximately 50 mole percent of at least one dianhydride residue and the remainder of a first diamine residue and at least an additional diamine residue, with the first diamine residue formed from a diamine containing a crosslinking reaction site. The crosslinking reaction site on the first diamine may include, but is not limited to, hydroxyl, thiohydroxyl, acrylate, methacrylate or combinations thereof. More preferably, the first diamine residue is formed from a diamine having a crosslinking reaction site containing a hydroxyl. When the polyimide cladding comprises a first diamine residue that has been formed from a diamine compound containing an acrylate and/or methacrylate crosslinking site, the polyimide cladding may be photo-crosslinked by methods determinable by those skilled in the art, such as by adding a UV sensitizer and exposing the film to UV light (actinic irradiation), or thermally crosslinked by adding a conventional free radical initiator such as a peroxide or azo compound. When the polyimide cladding comprises a first diamine residue that has been formed from a diamine compound containing a hydroxyl and/or thiohydroxyl, the polyimide cladding may be thermally crosslinked by the addition of an additive, also called a xe2x80x9ccrosslinking agent,xe2x80x9d such as an epoxy compound. In situations where the polyimide comprises either an acrylate and/or methacrylate in one diamine residue and also includes either a hydroxyl and/or thiohydroxyl in another diamine residue, the polyimide cladding may be both photo-crosslinked and thermally crosslinked, if the appropriate photosensitizer and crosslinking agent are added.
Proportionately, the polyimide cladding or core of the present invention comprise a preferred amount of the first diamine residue of from about 2 mole percent to about 15 mole percent, which possess a crosslinking reaction site. At least one additional diamine residue also is preferably present. Additionally, the combined diamine residues within the formed polyimide comprise a substantial amount of meta-linked diamine residues. Preferably the polyimide comprises at least about 25 mole percent of meta-linked diamine residues, more preferably, about 40 mole percent of meta-linked diamine residues, and most preferably approximately 50 mole percent of meta-linked diamine residues. A substantial amount of meta-linked diamine residue provides the polyimide with a sufficient amount of a meta-linked components such as to lower the birefringence of the polyimide to measurements of 0.01, 0.005, 0.001, 0.0005, 0.0002 and below.
Examples of the meta-linked diamine used in forming the first diamine residue non-exclusively include such compounds as: 
Preferably the diamine compound used to form the additional diamine residue of the present invention has a meta-linked configuration. Examples of the additional diamine of the present invention include without limitation such compounds as: 
wherein R is an aliphatic hydrocarbon having from about 2 to about 12 carbon atoms, an aliphatic hydrofluorocarbon having from about 2 to about 12 carbon atoms, or a polyalkylene oxide having from about 2 to about 12 carbon atoms. Preferably, the additional diamine residue is formed from a diamine corresponding to one of the listed structures above wherein R is an aliphatic hydrocarbon containing from about 3 to about 6 carbon atoms, an aliphatic hydrofluorocarbon containing from about 4 to about 8 carbon atoms, or a polyalkylene oxide containing from about 4 to about 6 carbon atoms.
In the most preferred embodiment, the polyimide cladding of the present invention is prepared from a first diamine of the structure 
an additional diamine having the structure 
and a second additional diamine having the structure 
The preferred amounts of the first diamine residue range from about 2 mole percent to about 49 mole percent, more preferably from about 2.5 mole percent to about 20 mole percent, and most preferably from about 3 mole percent to about 10 mole percent. The preferred amounts of one or more additional diamine residues range from about 5 mole percent to about 48 mole percent, more preferably from about 30 mole percent to about 48 mole percent, and most preferably from about 40 mole percent to about 47 mole percent.
The polyimide is prepared from one or more tetracarboxylic acid dianhydrides, also simply called dianhydrides, in an approximately 1:1 mole ratio with the total combined amount of the first diamine and any additional diamines. The dianhydride residue present in the polyimide of the present invention is preferably formed from dianhydride compounds such as: 
wherein X is a hexafluoro-isopropyl, sulfone, ether, thioether, or an isopropyl group; Y and Z are independently an ether, thioether, or an ester group, when R is an aliphatic or aromatic hydrocarbon having from about 2 to about 12 carbon atoms, an aliphatic hydrofluorocarbon, a fluorocarbon having from about 2 to about 12 carbon atoms, a polyalkylene oxide having from about 4 to about 8 carbon atoms; and Y and Z are independently hexafluoro-isopropyl, sulfone, ether, thioether, or isopropyl groups, when R is an aromatic hydrocarbon. The dianhydrides are not formed from BPDA, PMDA, and the like which increase the birefringence of the polyimide.
Generally, the total diamine and total dianhydride components are present in a molar ratio of approximately 1:1. The chain length may preferably be restricted by adding a predetermined quantity of a monoamine or a dicarboxylic acid anhydride to the reaction mixture. As is well known by those skilled in the art, the diamine and dianhydride are mixed together in a dry solvent at room temperature for several hours to form a poly(amic acid). This reaction mixture is then heated to about 170xc2x0 C. for several hours to form the polyimide. The reaction for the formation of the polyimide from the polyamine acid generally takes place at from about 160xc2x0 C. to about 300xc2x0 C. Normal precautions involving the use of dry solvents and monomers and the removal of water formed during the condensation reaction will lead to an increased molecular weight, which is normally desirable.
When desired, the polyimide may also be blended with a crosslinking agent. The blending may be done in a solvent at room temperature. Crosslinking agents such as epoxide compounds, e.g., diglycidyl ether of bisphenol A; oxazoline compounds, e.g., 1,3-phenylene dioxazolene; and isocyanate compounds, e.g., toluene diisocyanate, may be used. These compounds generally have two or more reactive groups.
The refractive index of polyimide cladding or core measured with light having a wavelength of 1.3 microns is preferably from about 1.45 to about 1.65, more preferably from about 1.50 to about 1.62, and most preferably from about 1.55 to about 1.61. The refractive index is controllable with the selection of the meta-linked diamines residues and dianhydrides residues within the polyimide. The refractive index may be lowered by the incorporation of specific diamines and/or dianhydrides in the polyimide, with the selection of particular diamines and/or dianhydrides being determinable by those skilled in the art, such as fluoro-containing monomers and aliphatic-containing monomers. Conversely, aromatic monomers increase the refractive index.
A controllable, or adjustable, cladding or core refractive index is desirable, given that it does not substantially change the birefringence or increase light scattering. In controlling the refractive index of the cladding or core, the polyimide composition of the cladding may be varied to suit a variety of different core materials that possess different refractive indices, and the core may be varied to suit a variety of different cladding materials that possess different refractive indices. The refractive index of the polyimides of the present invention can be controlled by selection of the types and amounts of meta-linked diamines and/or dianhydrides. Controllable refractive index and birefringence of the optical waveguide provide a flexibility in the most efficient use of a waveguide. The controllable polyimide layers permit construction of a specific refractive index ratio between the core material and the cladding material of the present invention.
In addition to controllable birefringence and refractive index, the polyimide cladding and/or core of the present invention provide thermal stability, increased solvent resistance, and low optical loss. Thermal stability of the polyimide cladding allows the present invention to be used in a variety environmental conditions. Low optical loss of the core and/or cladding of the present invention decreases the amount of energy required to sending a signal through a waveguide incorporating the present invention.
An optical waveguide subsystem as described herein may be fabricated by the process having the steps of (a) forming a polyimide from about 2 mole percent to about 15 mole percent of a first diamine, wherein the first diamine possesses a crosslinkable site, from about 35 mole percent to about 48 mole percent of at least one additional diamine, wherein the combined first and additional diamines possess a substantial amount of meta-linking configuration and the combined mole percent of the first and additional diamines equals approximately 50 mole percent, and approximately 50 mole percent of a dianhydride; (b) forming a solution of from about 20 weight percent to about 50 weight percent of the formed polyimide with from about 50 weight percent to about 80 weight percent of a solvent, and optionally from about 0.5 to about 10 weight percent of a crosslinking agent; (c) filtering the formed solution through a submicron filter; (d) spin-coating the filtered solution at from about 200 rpm to about 8000 rpm to form a film on a substrate; and, (e) baking the film at a final temperature which is the higher of from about 5xc2x0 C. to about 30xc2x0 C. above the glass transition temperature of the polyimide or approximately about 220xc2x0 C. for time period of from about 30 minutes to about 240 minutes. It is understood by those skilled in the art that in order to remove solvent from the film without forming bubbles or cracks before the final baking temperature, the film must be baked at lower temperatures for various lengths of time. The exact time-temperature process is determined for each sample in order to eliminate pinholes and blisters.
There are many solvents suitable for spin-coating the polyimide on a substrate that may be used for the present invention. Exemplary solvents non-exclusive include N-methyl pyrrolidinone (NMP), dimethylformamide (DMF), dimethyl acetamide, diglyme, ethyl lactate, and/or combinations thereof.
When the cladding comprises the crosslinked polyimide of the present invention, it may be used in combination with other waveguide materials. Accordingly, the core material may be a polymer, an inorganic glass, a semiconductor, and/or other like materials. Alternatively, the core materials can be polyimides of the present invention wherein the composition of the respective polyimides is such that the core polyimide has a higher refractive index than the cladding, and the cladding may be a polymer, an inorganic glass, a semiconductor, and/or other like materials.
Other additives may also be added to the polyimide depending on the purpose and the end use of the polyimide core or cladding. Examples of these include antioxidants, photostabilizers, volume expanders, dyes, free radical scavengers, free radical initiators, catalysts for crosslinking, contrast enhancers, nitrones, and UV absorbers and sensitizers. Additives may be included in amounts of from about 0 wt % to about 6 wt %, preferably from about 0.1 wt % to about 1 wt %. The additive should not phase-separate from the polyimide, as this may form optical scattering sites.
The present invention maybe formed on the surface of a suitable substrate. The substrate may be any material on which it is desired to establish a waveguide including semiconductor materials such as silicon, germanium, zinc selenide, aluminum nitride, silicon dioxide, gallium arsenide, silicon nitride, glass, quartz, plastics, ceramics, gold, crystalline materials and the like. The substrate may or may not contain other devices, either topographical features such as grooves or electrical circuits or electro-optic or photovoltaic devices such as photo detectors. For devices where the light transmissive region is an organic layer of lower refractive index than the substrate material, it is necessary to first place another layer of material having a refractive index lower than the refractive index of the waveguide material between the substrate and waveguide. Such a layer is known in the art as a buffer layer or a cladding layer and may be comprised of, for example, a semiconductor oxide, a lower refractive index polymer or a spin-on silicon dioxide glass material.
The present invention may be used in either a slab, rib or channel waveguide. A slab waveguide is one in which the optical wave is confirmed only to the plane of the film. Channel and rib waveguides are those in which the optical wave is also confined laterally within the film.
Channel or rib structures are necessary for many passive and electro-optic devices because they allow the light to be directed to certain areas of the substrate as well as providing a mechanism for splitting, combining optical waves and coupling light from the waveguide to optical fibers.